Vinyl chloride resins are less expensive and suitable to general purposes and, therefore, is being produced and consumed in a huge amount. However, vinyl chloride resins have drawbacks that they have low impact resistance and are difficult to process.
It is known to add a methyl methacrylate-butadiene styrene copolymer (or MBS) as an impact modifier in order to improve the impact resistance of vinyl chloride resins (Japanese Patent Application Laid-Open Nos. Sho-57 98542/1982, Sho-58-152039/1983 and Hei-2-191652/1990). However, flowability of such a vinyl chloride resin composition decreases with the increasing amount of MBS added, so that a load in kneading increases.
In order to improve the processability of vinyl chloride resins, it was tried to add a lower molecular weight vinyl chloride resin, to modify vinyl chloride resins with ethylene or analogues, or to adjust the amount of a plasticizer or lubricant added. However, the addition of the lower molecular weight resin or a plasticizer causes significant decreases in softening point, impact strength and tensile properties of the vinyl chloride resin composition and sometimes may result in setting of a molded product in a process and bad surface appearance. In the ethylene modification, a high content of ethylene or a low degree of polymerization is required to improve processability at lower temperatures, which however results in deterioration of mechanical properties, particularly, softening point and impact strength. Meanwhile, to raise impact strength, a high degree of polymerization or addition of an impact modifier such as MBS is required and, then, processability will worsen. Use of a large amount of a lubricant causes bleed-out or plate-out during molding and bad surface appearance, and sometimes may result in deterioration of mechanical properties due to insufficient kneading.
It was also tried to diversely change production conditions for polyvinyl chloride molding compounds so as to improve processability of the compounds. A method which relies on control of kneading conditions in an extruder requires skilled labor in general. Such control is difficult in vinyl chloride resins which are difficult to knead. Further, in conventional rigid polyvinyl chloride molding compounds, process properties depend largely upon changed production conditions such as temperatures and types and dimensions of a screw. Accordingly, molding conditions must be changed for each compound obtained and, sometimes, molding is even impossible.
As mentioned above, it was difficult to maintain or improve impact properties of vinyl chloride resins while minimizing deterioration of other physical properties and, at the same time, to improve processability.